Project 1 Determinants of disease progression and role of latent infection in transmission will study a series of factors that will help understanding why only a limited number of individuals infected with Leishmania donovani develop Visceral Leishmaniasis (VL) on the Indian subcontinent. Project 1 will also contribute to assess the role of asymptomatically infected individuals (those who do not progress to disease) in L. donovani transmission. This project will combine field and laboratory work. Two serological surveys of leishmaniasis infection will be done in high transmission areas to identify recent asymptomatic seroconvertors and controls in the study area (see Core C for details). Similariy, VL cases and controls (i.e healthy household contacts) will also be identified and included in the different studies. Project 1 is divided in 4 specific aims. Aim 1 will examine the association of HLA-type with seroconversion and disease progression controlling for several confounders identified in the previous risk factor studies. Aim 2 will use a recently developed quantitative PCR to assess parasite load in different subpopulations (i.e.seroconvertors and seronegative individuals) and assess its influence in progression to VL taking into account genetic and environmental factors. Aim 3 will examine the association between co-infections with other Neglected Tropical Diseases (NTD) and VL in two case-control studies. (1) An unmatched case-control study comparing Leishmania seropositives with the general population and (2) a matched case-control study comparing VL cases to community controls will be used to assess the effect of filaria- or geo-helminths-L. donovani co-infection on progression to VL. Finally, Aim 4 will contribute to study the role of asymptomatically infected persons by (1) validating a modified SLA-based Quantiferon to detect cellular immune response in asymptomatic individuals and (2) selecting individuals (peripheral blood mononuclear cells (PBMC) culture positive) to be included in the xenodiagnosis experiments (see Project 2 for details) .